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Has anyone tried running ducts through 3-piece crown molding or coffered ceiling?

whitenack | Posted in Interior Design on

Hi all,

I am trying to study my lesson before talking with some HVAC pros about ductwork. I am building a 2 story house with a central hall plan. I think a compact duct system is very do-able, because I have a walk-in closet nearby to place the air handler and the ducts won’t have very far to go to serve all the rooms. Hoping that a ducted mini split will get the job down so the air handler noise is not a problem.

What I don’t have clear in my head is the best way to route the ductwork.

Option 1…The 2nd floor will have 9 ft ceilings, so I have room to drop the ceiling in the hallway, but I’m not sure how big the ducts have to be and I don’t want to drop them down so much it detracts from the overall look of the house.

Option 2… My builder suggested creating a chase in the attic, sealing it up with spray foam and then mounding up the cellulose over top of that. My concern with this is a.) I know this option is less efficient than keeping the ductwork entirely below the ceiling, b.) the bottom of the ducts would have to sit about 12-14″ above the ceiling plane to clear a structural beam running perpendicular to the joists. And that 12-14″ is where the bottom of the ducts would be, depending on the dsize of the ducts, the top would be much higher, plus enough room to cover over top with cellulose. This means a large mound of cellulose to encapsulate below and above the duct, and c.) I also would like to have a whole-house fan mounted in the ceiling plane, and not sure if there would be room if there is a huge mound of insulation needed to cover the duct.

Option 3?? Has anyone tried hiding the ducts in a cofferred ceiling or some high-style crown molding? I see plenty of suggestions for building a soffit along the wall, but I’m not sure I like the look of a plain soffit. Those 3-piece crown moldings are really just a soffit with molding above and below, but wondered if anyone had tried to run ductwork through them. I guess it depends on how big the ducts have to be.

According to my own (and subject to error) Manual J calcs, the total upstairs cooling load is 14k, with only 4k or so needed down the hall. So hopefully that means that the ducts don’t have to be that big. And I guess another option (#4?) would be to just put a ductless head on the other end of the hall to serve the two bedrooms down there, as long as the occasionally-closed bedroom doors won’t be a problem.

I know I need to have someone do some Manual S, T & Ds to know for sure, but I’d like to know ahead of time so I can help the HVAC guys think outside the box.

Thanks folks.

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  1. GBA Editor
    Martin Holladay | | #1

    I hate to tell you -- I bet you suspected what I was going to write -- but you can't decide how to hide your duct system until your duct system has been designed. You need a room-by-room heat loss and cooling load calculation; you need to determine the distances from the ducted minisplit to the registers or diffusers, and you need to determinethe number of elbows required to get there; and you need to choose your equipment (so that we have some cfm and static pressure specs).

    Almost undoubtedly, a 9-inch-deep dropped ceiling in your hallway is the best way (and the easiest way) to go.

  2. charlie_sullivan | | #2

    What climate are you in? A whole-house fan can sometimes do more harm than good.

  3. whitenack | | #3

    Martin, Haha, Yes, I suspected you were going to say that...that's why I tried to qualify it as best I could by acknowledging the S, T & D still needed to be done. I'm trying to be patient and wait for that to be done but I have trouble being patient. This is the last piece of the puzzle in trying to build this as efficiently as possible and once I know the details I can finally rest. But you may have told me what I really need to know (9-inch deep dropped ceiling). I assume that is 9" total, or a 9" cavity plus the framing below that? The question I have to answer, then, is whether an 8' 3" ceiling is good enough for the hallway.

    Charlie, I am in central KY, climate zone 4A. Whole-house fans aren't a substitute for A/C in our humid summers, but they are very nice in the spring and fall when the days get up to the 70s-80s but the nights get down in the 50s-60s. Or, if you grew up poor like I did, it actually can be the sole source of cooling. I'll never forget making my bed in the threshold of my bedroom door because that was the coolest place in the room.

  4. mpg9999 | | #4

    Check out for some good information on the different strategies for keeping the ducts inside the envelope.

  5. GBA Editor
    Martin Holladay | | #5

    I was assuming an 8 ft. ceiling and a 9-inch deep (clear measurement) soffit. There is nothing wrong with an 8 ft. ceiling for a hallway.

  6. GBA Editor
    Martin Holladay | | #6

    GBA also has a "ducts inside" article: Keeping Ducts Indoors.

  7. whitenack | | #7

    To play devil's advocate, what kind of materials would be needed for an insulated duct run through the attic? If you had to run the ducts, say, a foot off the attic floor to avoid the framing obstacles, how would you build the chase? A simple box made of plywood/osb/polyiso all sealed up and then wrapped/buried in a bunch of insulation? What r-value would be needed? R60 all the way around just like the rest of the attic? Or more since the temperature differences between the ducts and the attic air would be more than the difference between the indoor house air and outdoor air?

  8. JC72 | | #8

    Depending on the room layout have you considered very deep open web floor trusses? They will give you the ability to run duct between the floors and ceiling without having to break out into the attic.

  9. GBA Editor
    Martin Holladay | | #9

    It's theoretically possible to build an insulated box to enclose attic ducts, but almost everyone who tries to do this fails in reaching their goals.

    First, the ducts must be in excellent condition, with carefully sealed duct seams. Once you build this box, you won't be able to inspect the ducts again.

    The delta-T between the air in the ducts and the attic air is larger than the delta-T between the conditioned air in your home and the attic air. If the attic floor has R-60 insulation, then (logically) the box you are building should have insulation that exceeds R-60. Will anyone really do that? Probably not.

    But if you want to try -- the idea is to build an airtight box out of OSB or rigid foam, and to surround this box with R-60 insulation. Fifteen inches of EPS would work. (I don't recommend that you actually do this.)

    Once you look at all the ducts in your attic, you will probably conclude that it's easier to just create an unvented conditioned attic.

  10. whitenack | | #10

    Sounds like I need to build a box with 10" of polyiso. Haha. Thanks for all the help folks, I've got some studying to do.

  11. LucyF | | #11

    I don't know much about ducted mini splits, but I do have some advice on HVAC in the South.

    First question, how involved are you going to be in the actual building of the house? If the answer is very, then you can ensure that the duct will be sealed properly because you will be there or you will be doing it.

    If someone else is doing it, only allow mastic or real air sealing tapes (not the infamous duck tape) probably imported from Europe that you must inspect before insulation. If you request true metal ducts, no flex duct, the installers will probably want to use a fiberglass insulation sleeve that is supposedly R-6 (really?) that compresses down to an inch. In other words, with perfect air sealing it is maybe an R-3 or 4. You can't do much about that, but just know that the ducts aren't really insulated at that point.

    The other thing that the installers will want to do is install the registers on an outside wall under a window. Absolutely insane in my opinion. Why do you want your conditioned air to go in the place where it will lose heat or heat up faster? Crazy thinking. In a well-insulated house, you can put it anywhere and the room will feel comfortable, meaning shorts runs are what you want.

    The only house building I've been involved in is my brother's house. I worked really hard to get them to understand about the ducts not needing to be under a window or the far side of a room, but they couldn't even help themselves and they still installed one or two under windows which was a pain - because it was my job to insulate the ducts underneath the house on a pier foundation. I must have spent 100 hours crawling under the house taping the ducts, then taping the 1.5" polyiso box around the ducts.

    Martin is correct (as is almost always true) - the foam box routine is very difficult and very difficult to do properly. I'm not sure it's correct yet at my brother's house. I have to open up some areas to see if there is any condensation when the AC is running. I left a little taped up window with a polyiso flange that I can cut open and reseal to check the ducts.

    Good work on asking these questions now. Even though it's a pain to contemplate these issues - you are saving yourself future headaches, time and even money.

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